Over the last decade the fraction of energy added to power grids by wind farms has increased significantly. Therefore, there is also an increased need for Controlling wind farms delivering energy to Utility grids with respect to a number of parameters which are important for the Utility grid to work faultlessly. In addition to the voltage and the frequency of the power delivered to the Utility grid, it may also be important to provide, on request of the Utility sys-tem Operator, a certain reactive power or a certain power factor. The reactive power is the power needed by inductive and capacitive users to build up their magnetic and electric fields, respectively. With an alternating current such magnetic and electric fields will be built up and down periodically, which leads to a reactive power flow from and to the electric generators. The power factor is the cosine of the phase angle between voltage and current.
Variations in the demand for reactive power in the Utility grid need to be compensated through reactive power provided by the power generators. As long as the power delivered to the Utility grids by wind farms was only a minor fraction of the total power delivered to the grids, there was no need to react to the reactive power demands or power factor demands of the grid by the wind farms. However, with increasing fraction of power fed into Utility grids by wind farms, the dynamic power factor control or a dynamic reactive power control of wind farms becomes more and more important.
Dynamic power factor control of wind farms is often implemented with capacitors banks mounted in individual wind turbines. Capacitors are switched on and off the grid to provide such reactive power as is required to meet the specified power factor. Dynamic power factor control may also be implemented by an arrangement where the individual wind turbines are equipped with a power electronic Converter that converts part or whole of the electricity supplied by the wind turbine. The power electronic Converter of the individual wind turbine is programmed to control the power factor of the electricity supplied by the wind turbine.
For both types of power factor control System the desired power factor is typically provided as a signal from a central SCADA (Supervise Control and JData Acquisition) System. The Utility System Operator dispatches a request to the wind farm for a certain power factor and the central SCADA System forwards the power factor request to the individual wind turbines, either directly or after modification, to compensate for the power factor contribution of the wind farm electrical infrastructure. The power factor is controlled locally at the individual wind turbines by adjustment of the power factor of the electricity supplied by the individual wind turbines to correspond to the power factor requested by the SCADA System. Such an arrangement for dynamic power control is, e.g., de-scribed in U.S. Pat. No. 5,083,039, which describes a wind turbine with dynamic power factor control, sending control signals to power electronic Converters of wind turbines. The power electronic Converters are then locally controlled such that the power factor delivered by the local wind turbine is shifted through changing the ratio of active and reactive current supplied to the grid by the inverter module of the power electronic Converter.
A dynamic power factor control system as outlined above requires a SCADA system with functional and fast-reacting connections to the individual wind turbines. If the individual communication of the wind farm is slow or deficient, the dynamic power control will not function faultlessly. Further, unless all turbines are operating at the same active power output, which will rarely be the case if the wind speed is not sufficiently high to cause all wind turbines of the farm to operate at rated capacity, the reactive power supplied by an individual wind turbine will change proportionally with the active power supplied by the individual wind turbine. This means that some wind turbines will provide significantly larger proportion of reactive power than others, which in turn leads to current flow in the wind farm that is less balanced and causes higher losses than what could be achieved with more balanced current flow.
L. Holdsworth, et al. describe “a direct solution method for initializing doubly-fed induction wind turbines in power system dynamic modeis” in EE Proc.-Gener. Trasm. Distrib., Vol. 150, No. 3, May 2003. In this model, the wind turbine is represented as a PQ bus. The model uses a control strategy of Optimum power extraction (speed control) and local power factor correction. The results of the direct solution method are the injected rotor voltages for Controlling the wind turbine. Moreover, a model in which the control strategy is modified to terminal voltage control is described. In a terminal voltage control, in contrast to power factor or control reactive power control, the output voltage of the wind farm is con-trolled to value requested by the utility System Operator.